An electromagnetic relay generally comprises an iron core having solenoid wound thereon, a yoke connected to the iron core at its one end so as to define a magnetic gap therebetween, an armature hinged to the yoke so as to be driven in the direction to close the magnetic gap when the solenoid is energized, a spring member urging the armature away from closing the magnetic gap, a contact mechanism, and transmission means for converting the displacement of the armature to that for actuating the contact mechanism.
In such an electromagnetic relay, since the magnetic gap is desired to be minimized so as to produce a maximum attractive force between the iron core and the armature and a certain actuation stroke is required for reliable actuation of the contact mechanism, it is desirable to attain a fairly large lever ratio or transmission ratio with the transmission means. However, to attain a large lever ratio, the size of the transmission means, for instance, consisting of a lever mechanism tends to be increased beyond an acceptable limit.
The lever ratio may be increased by using a longer lever, but this will increase the size of the electromagnetic relay. Therefore, it is desired to increase the lever ratio substantially without increasing the size of the electromagnetic relay. It is conceivable to achieve this goal by providing a notch in the yoke for receiving a lever arm extending from the armature so as to curb the increase in size by thus receiving the lever arm within the general contour of the yoke while ensuring a sufficient length to the arm, but the resulting diminution of the width of the yoke may reduce the mechanical strength of the yoke to an unacceptable level. In particular, susceptibility of the yoke to deformation makes its handling extremely troublesome particularly in the case of small electromagnetic relays. Further, reduction in the cross sectional area of the yoke increases the magnetic resistance of the yoke which forms a part of a magnetic circuit and the magnetic efficiency of the magnetic circuit tends to be impaired. It means an increased power consumption and a reduced sensitivity.
Electromagnetic relays for high frequency current requires a shield structure for its contact pieces. Therefore, assembly work tends to be highly troublesome and it has been desired to design a highly compact structure for high frequency electromagnetic relays which is easy to assemble. In particular, the shield case must be a highly enclosed structure in order to achieve a high isolation capability, but this in turn causes an increase in the manufacturing cost and the possibility of thermal deformation during the process of soldering lead wires to the ground terminals integrally provided in the shield case. Also, the shield case is desired to be manufactured by a simple stamping process for reducing manufacturing cost.
As contact structures for an electromagnetic relay of the aforementioned type, there are known the cross bar contact structure in which a moveable contact member and a fixed contact member both consisting of planar pieces contact each other by a line, and the rivet contact structure in which a planar piece and a hemishperical projection contact each other by a point. The cross bar structure has the disadvantage that the contact tends to be unstable and a sufficient contact pressure may not be attained. On the other hand, the rivet contact structure can ensure a sufficient contact pressure, but since the contact is made by a point, in case of any slight defect in the contact area, the state of contact becomes poor and, therefore, durability and reliability may not be sufficient.